The Boko Haram sect has claimed responsibility for bombings and attacks in which 38 people died. Photograph: Afolabi Sotunde/Reuters

The radical Boko Haram Islamist sect has claimed responsibility for the Christmas Eve bombings and church attacks in Nigeria in which at least 38 people died.

The group threatened new attacks to avenge violence against Muslims in and around Jos, where religious fighting has left more than 500 people dead this year and displaced many others.

Authorities had already blamed Boko Haram for some of the deaths, and the online statement attributed to the group warned: "We will continue with our attacks on disbelievers and their allies and all those who help them."

It was not immediately possible to contact members of the sect, which had not previously used the website on which the statement appeared. The head of national police, Yemi Ajayi, said the authorities were still investigating.

Two bombs went off near a large market in Jos where people were doing last-minute Christmas shopping on Friday. A third hit a mainly Christian area, while the fourth was near a road leading to the city's main mosque.

On the same day, two churches in the northern city of Maiduguri were attacked, killing at least six people.

Nigerian authorities blamed Boko Haram and said a Baptist pastor and two choir members preparing for a late-night carol service were among the victims.

Religious violence in Jos and neighbouring towns and villages has not typically been linked to Boko Haram.

Nigeria, a country of 150 million people, is almost evenly split between Muslims in the north and the predominantly Christian south. Jos is in the nation's "middle belt", where dozens of ethnic groups vie for control of fertile lands.

The Jos violence, though fractured across religious lines, often has more to do with local politics, economics and rights to grazing lands.

The government of Plateau state, of which Jos is the capital, is controlled by Christian politicians who have blocked Muslims from being legally recognised as citizens.

That has locked many out of prized government jobs in a region where the tourism industry and tin mining have collapsed in recent decades.